Aflatoxin reduction goal for corn

Texas researchers and Extension specialists are mounting a full-court press on aflatoxin contamination in corn, hoping the combined efforts of plant breeding, biological control measures and post-harvest remediation will reduce the negative effects the costly fungal infection creates for the Southwest corn industry.

Murray said plant breeding will be “the No. 1 solution to aflatoxin,” in a multi-disciplined approach that also includes best management production practices, field-applied atoxigenic strains of A. flavus and remediation (identifying binders that prevent toxin absorption).

Food science and veterinary departments also will help with nutritional efforts.

“Aflatoxin (Aspergillus flavus) is a toxic compound and a potent carcinogen,” Murray said. “Any contamination above 300 parts per billion must be destroyed. Acceptable level is 20 parts per billion.” Other restrictions also apply. Corn with contamination levels between 20 parts per billion and 300 parts per billion is restricted to animal feed.

To survive aflatoxin requires a disease triangle — spores, proper environment and a host. Favorable conditions include drought and heat stress. Insect infestation, especially Lepidoptera, also favors aflatoxin contamination. Late planting also may be a factor.

He said sampling over the past few years has identified high concentrations of the toxin in certain “hot zones” in Texas.

Murray said in 2008 insurance payments to Texas farmers for aflatoxin “exceeded $14 million and there was greater economic damage that cannot be quantified.”

“Plant breeding will be a key,” he said.

Most elite corn germplasm is susceptible to aflatoxin contamination. “Breeding is one of the least expensive methods of dealing with the issue. But so far we have not identified anything with complete resistance. We are testing in multiple locations and have found some lines with reduced levels of aflatoxin.”

He said several hybrid traits may be important, including husk coverage and fit. “Larger husks seem to be more resistant and a tight-fitting husk also seems to be an advantage. But most hybrids are developed in the Midwest and they prefer a looser husk.”

He said maturity, flowering time and days to harvest may be factors as are kernel hardness, ear nod (or ear droop), earworm resistance and drought tolerance. “Our challenge is the large number of traits we need to select. Deleting genes may be a possibility.”

Murray said efforts will include both traditional and genomic-assisted breeding.

Researchers are also looking at binders to apply to contaminated corn. These binders, such as smectite clays, that will prevent absorption of the aflatoxin.

“We are also investigating improved spectroscopic detection, using food science applications to evaluate cooking process interactions to reduce or destroy aflatoxin, and international work evaluating binder reductions of aflatoxin in human foods.”

Isakeit said transgenic hybrids with insect resistance traits showed “no effect (on aflatoxin contamination) but hybrids do make a difference, and anything that helps control insects is a good idea. The hybrid selection is important and there are differences between hybrids. How well adapted a hybrid is to a certain location is a key.”

Smith and Isakeit also discussed atoxic strains of aflatoxin that outcompete the toxins and may reduce contamination levels significantly. Smith discussed Afla-Guard, a Syngenta product and a “non-toxic strain of Aspergillus flavus.”

Growers would inoculate the crop with Afla-Guard at a certain time after planting to reduce potential for the toxic strain to develop. Smith said the process involves coating whole barley seeds with spores of the non-toxic strain and applying those spores to the field.

“Spores need moisture to germinate and typically do so within a week. We’ve also conducted trials with aerial application and found the granules in the corn whorl, where spores will develop from the morning dew in the whorl or sheath. That’s also closer to the target we’re trying to protect.”

Smith said the product was developed by a USDA scientist in Georgia and first tested on peanuts. Another strain has been developed by a USDA scientist in Arizona.

Smith said the product resulted in 85 percent aflatoxin suppression in peanuts. Recent tests in Texas corn also show significant reductions in aflatoxin levels. Smith said on-farm trials in Clay, Whitewright, Tom Bean County and Dorchester showed significant reductions.

Testing in 2007 and 2008 indicated average reductions from 81 percent to 90 percent.

Smith said lowering contamination levels that much will improve marketability of Texas corn. “We don’t know each year what aflatoxin contamination levels will be,” he said. “We can’t determine the weather. We may know that certain fields are more likely to have aflatoxin, but weather is a major factor.”

He said the atoxic strain may overwinter, “but we’re not certain if it will produce enough spores to be as competitive as we want it to be.

Smith said Afla-Guard cost is not known yet. “The label is into EPA.” Application rates have been from 10 to 20 pounds per acre.

He said reduced aflatoxin levels will open “access to premium corn markets.”

Isakeit said Afla-Guard and the Arizona strain, AF-36, developed by USDA scientist Peter Cotty, both hold promise for reducing aflatoxin contamination in corn.

“It needs two to three days to sporulate and it needs moisture. Timing is critical. It should sporulate before corn flowers.”

He said risk is greater with dryland corn because of uncertain moisture. “(In dryland) we may need to apply earlier. Also, heavy rain may wash the fungus into the soil and it will not push through as seed would.”

He agrees with Smith that aerial application may be a promising alternative to take advantage of dew moisture captured in the whorl or sheath.

Some of his tests have resulted in contamination levels dropping from 33 parts per billion to 6 parts per billion in 2007 experiments. That test was conducted in a field with little toxic strain evident.

In 2009 he inoculated a field with the toxic strain. He applied the atoxic strain at silking and the aflatoxin five days later. The field was pre-irrigated to provide adequate moisture. Results were significant, he said, but not as high as other tests.

“The control plot registered 152 parts per billion and the Afla-Guard plot had 88 parts per billion. That’s a 42 percent reduction.”

Not all tests have done that well. He said an on-farm test on subsurface drip-irrigated corn showed “no effect. The material probably was applied too late."

Aerial application on a Williamson County trial brought aflatoxin levels down from 102 parts per billion in the check to 25 parts per billion on the treated area. “This was under extreme drought stress,” Isakeit said.

Getting a handle on aflatoxin would be a boon to Southwest corn growers, Smith said, as well as producers and consumers worldwide. “According to the Food and Agriculture Organization, 25 percent of the world’s agricultural production is contaminated by mycotoxins,” he said. “The economic losses in the grain industry in the USA alone are estimated to be at least $923 million annually. It is a serious concern to downstream food and feed companies and may limit growers from high value markets and result in significant discounts and lost profitability.”

Some estimates suggest aflatoxin costs U.S. farmers more than $200 million a year.

“We hope research efforts will minimize and eliminate aflatoxin risk to end user consumers of corn,” Murray said.